In aviation it is important for safety and economic reasons that the aircraft be properly loaded. An aircraft should not attempt to take off if the aircraft is too heavy or if the center of gravity is too far removed from the center of lift. Furthermore, in the commercial operation of aircraft it is desirable for economic reasons to load the aircraft with as much revenue-producing cargo as possible. Therefore, a very accurate and reliable measure of weight and balance is needed to permit loading of an aircraft to maximum capacity with assurance that safety is not compromised.
Accurate loading typically entails making sure that the gross weight of the aircraft is within acceptable limits and that the load is properly distributed. The measure of the load distribution is primarily determined by locating the longitudinal center of gravity (CG) for the aircraft. The CG should typically be within a certain range for a given load as recommended by the manufacturer and/or the Federal Aviation Administration.
The longitudinal CG may be determined by considering the weight at locations along the aircraft and using the moment (weight.times.distance) developed about a given reference line or datum. The sum of the moments is then divided by the total weight to arrive at one point where the plane is balanced or could be supported, which is the CG. The CG is frequently expressed in terms of percentage mean aerodynamic chord (% MAC).
Early in aviation, the importance of weight and balance was realized. To arrive at the weight under one conventional approach, the pilots frequently begin with an empty weight for the aircraft as determined by the manufacturer and/or maintenance personnel and then add the weight of the luggage, fuel, and passengers and the like as they are loaded. This technique frequently involves estimating the weight of each loaded object and raises some uncertainty in the calculation of the weight and CG; for example, a standard weight may be assumed for each person, e.g., 160-180 lbs., notwithstanding that the actual passenger may be a small child under 100 lbs. Additionally, the technique may slow the departure of the aircraft as the pilots or personnel make calculations up to the last minute before departure.
A number of on-board weighing and CG determining devices have been proposed and developed. In the past few decades, several approaches to on-board aircraft weight and balance systems have been developed. Unfortunately, these systems have not enjoyed great commercial success and acceptance apparently because of shortcomings. Such shortcomings may have included inadequate accuracy and dependability. Additionally, some designs may be too expensive to manufacture or install.
Many of the shortcomings of on-board weighing and CG determining devices have been identified. For example, in a 1980 article, Dr. R. M. H. Cheng concluded that among the main problems with on-board weighing and CG devices are friction in the oleo landing gear struts and high noise to signal ratio in strain gauged systems. A.C. Macdougall and Dr. R. M. H. Cheng, "The Light-Weight System," SAWE Journal, 1980, pp. 41-46. Accounting for the friction in the oleo strut is considered a major problem, and some have attempted to address this problem.
Honeywell has developed a weight and balance system that avoids the oleo strut by placing a gauge in the gear lug. The Honeywell System typically has included landing-gear-mounted deflection sensors, a calibration module containing all gear parameter information, a computer unit, a pitch attitude sensor, a cockpit display, and a remote dedicated display unit for cargo loading. The sensor used by Honeywell is alleged to be covered by U.S. Pat. No. 4,269,070, entitled Strain/Deflection Sensitive Variable Reluctance Transducer Assembly, issued May 26, 1981 to Nelson, et al., and assigned to Weico Corp. The sensor allegedly measures shear deflection directly while ignoring bending and other deflections and includes inductive mechanism rather than resistive strain gauges. The sensor is mounted on the aircraft lugs of the landing gear or installed in the axles.
Modern aircraft frequently use landing gear designs that include a shock absorbing system including a small orifice plate within the cylinder of the landing gear strut and are refereed to as "oleo shuts." Hydraulic fluid is forced through the small orifice within the strut cylinder, and the orifice in conjunction with a metering pin, which varies the flow area of the orifice, damps the transient loads on the landing gear.
On the ground, the aircraft is supported by two forces produced in the landing gear strut: (1) force produced from the pressure of the fluids in the strut on the piston and (2) a force developed through friction associated with the strut piston in the strut cylinder. Though minor, the frictional forces are too large to disregard in precision weighing. The magnitude of the frictional forces is generally not repeatable because it varies due to a number of factors including temperature (due to the varying stiffness of seals), the length of time since moving the aircraft (due to squeezing out lubricating fluid), and the direction of last motion before coming to rest (due to deformation of seal members).
Landing gear struts typically include a strut piston and a number of O-rings used to provide a seal about the piston. The O-ring seals may cause significant friction in the cylinder about the piston. The frictional forces on the O-rings and piston have previously made it difficult to directly use the pressure in the strut cylinder to arrive at an accurate measurement of the weight experienced by the strut.
U.S. Pat. No. 5,214,586 entitled Aircraft Weight and Center of Gravity Indicator issued to Nance on May 25, 1993, describes an on-board system for use in measuring and computing and displaying the gross weight and center of gravity of an aircraft. A computer receives temperature and pressure information from the landing gear struts. The computer includes software that corrects or compensates for physical changes to strut components due to temperature fluctuations, drag, and hysteresis. The accounting for drag is based, however, on a drag component determined during a calibration process. A "drag to temperature" adjustment curve or "look-up table" is charted by recording various airplane weights while the airplane is on a calibration scale and comparing those weights with corresponding pressure readings through a wide-range of temperatures. This table is used to determine the weight and consequently the center of gravity for the airplane during operation.
U.S. Pat. No. 5,521,827 ("'827 Patent"), entitled On-Board Aircraft Weighing and Center of Gravity Determining Apparatus and Method issued to Lindberg, et al. and assigned to the owner of the present application describes a system that includes provisions for taking pressure measurements associated with an aircraft's oleo struts at different positions between an extended position and a retracted position. The '827 Patent also references an embodiment using dynamic friction in a limited manner. The '827 Patent is incorporated herein by reference for all purposes.